Treatment of (Z)-alpha-Fluoro-beta-trifluoromethylvinylstananes 3 with n-butyllithium gave tetrafluoropropene anion 4, which was reacted with alpha.beta-unsaturated aldehydes affording 6. With or without isolation of 6. DAST was added and reacted with 6 to give polyfluorinated alka-2E,4E isomers, exclusively. It is noteworthy that the migration of the double bond occurred only at the non-fluorine-containing one.